Installation

npm i -g evm2term

Usage

Usage: evm2term [options] <json file containing the ast>

Converts an Egg AST to a term representation

Options:
  -V, --version  output the version number
  -i, --indent
  -h, --help     display help for command

Description

Provides an executable evm2term that summarizes an AST stored in a json file. Currently only there is a description file egg-ast-description.js giving support to the ASTs provided by the egg compiler used in the classes of the subject Language Processors:

For instance, for this Egg program:

➜ cat cat examples/summult.egg

+(a,*(4,5))

The AST generated by the any parser is usually a long JSON:

➜  eggc examples/summult.egg       
➜  evm2term git:(generic) ✗ cat examples/summult.json

{
  "type": "apply",
  "operator": {
    "type": "word",
    "offset": 0,
    "lineBreaks": 0,
    "line": 1,
    "col": 1,
    "name": "+"
  },
  "args": [
    {
      "type": "word",
      "offset": 2,
      "lineBreaks": 0,
      "line": 1,
      "col": 3,
      "name": "a"
    },
    {
      "type": "apply",
      "operator": {
        "type": "word",
        "offset": 4,
        "lineBreaks": 0,
        "line": 1,
        "col": 5,
        "name": "*"
      },
      "args": [
        {
          "type": "value",
          "value": 4,
          "raw": "4"
        },
        {
          "type": "value",
          "value": 5,
          "raw": "5"
        }
      ]
    }
  ]
}

You can get the shape of the AST using evm2term:

✗ evm2term examples/summult.json
apply(op:word{"+"},args:[word{"a"},apply(op:word{"*"},args:[value{4},value{5}])])

The Term Language to Summarize ASTs

Term is a DSL to summarize ASTs. Here is an attempt to describe the language:

term -> ('NAME' ':')? 'TYPE' '(' term (',' term)* ')'
  | leaf
leaf -> ('NAME' ':')? 'TYPE' ('{' 'ATTRIBUTE' '}')?

• Token 'NAME' is the name of the child in the node,
• Token 'TYPE' represents the type of the node,
• The token 'ATTRIBUTE' is the JSON stringify of a single attribute of the leaf node.

To summarize the AST the following rules are followed:

• Only the type of the node is shown
• Only one selected attribute of a leaf is shown (Between curly brackets)
• Array n-ary nodes are allowed (and they go between brackets)

Trick

The syntax of the output seems to be legal ruby. You get syntax highlighting by saving the output with the .rb extension

Future Work

By adding a configuration JS file following the pattern in egg-ast-description.js, the program can be used to work with different ASTs.

• Write more AST descriptions for different JS transpilers (esprima, babel, etc.)